mains Hot water, and do I convert open heating to a closed heating system

hi all,
been trawling through the search engines for two days and decided I know less about the options I have than when I started.
The Problem: I currently have an open vented heated system, that is under speced from the boiler point of view and I am trying to suss out what is the best road map to change the system to what I want without ripping it all out in one go as I can not afford it.
Main issues to be sorted are: When the heating system runs the overflow overflows constantly Hot water is at too low pressure I would like to add a radiator in the loft at some point as it is a walk in room
Solutions ?
I am thinking that using a thermal store will give me the mains pressure hot water I want while still using the open vented system Installing an airseperator from what I read on the net should stop the overflow issue, but not found one that I can buy convert the system at a later date to either a closed system or buy a boiler that is already a closed system to get the hot water in the loft. Can you get such boilers that are not combi boilers or do I need to install the parts. If that's the case, do I need to put the expansion vessel near the boiler and hence in the kitchen..
How would others tackle the problem ? useful urls for suppliers like the air vent would be welcome, the ones I have found seem to be targeted at just removing the air
many thanks and cheers
ian
ian tracey snipped-for-privacy@webkiosk.co.uk
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The easy answer is something like the Baxi system boiler, this contains pump, air vent, safety valve, expansion vessel, and of course the boiler. just add two zone or one three port valve to connect to your system, and remove the existing cold feed and vent. IMHO a Megaflow is better than a thermal store not least because of the large volume of water to treat with inhibitor in a thermal store, also thermal store are less efficient due to there higher operating temp.
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Thanks MuddyPaw, can you be kind enough to explain the line:

is it because the higher temp = greater loss across the tank insulation ?
cheers
ian
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Not so. A heat bank, with a plate heat exchanger is more efficient than a Megaflow. They also can operate at very high cold mains pressure, where a Megalfow operates, typically between 1 and 3.5 bar. If you have 6 bar mains , a heat bank will allow 6 bar for hot water. A heat bank has no complex pressure reducing and blow off gear, high temp cut outs, large overflow pipes, tundish, having to service it each year, high pressure water storage (when they go it is a full insurance job for the house), etc
You can have only a small amount of hot water at the top of a heat bank and they will deliver useful hot water, which is not the case with an immersed coil thermal store. Heat banks and thermal stores are not quite the same thing. A heat bank can be DIYed, A Megaflow requires a BBA approved fitter. Expensive!!!
Many councils have been bitten by unvented cylinders (Megaflows). Constant callout for dripping overflows, etc, that they are going to heat banks. Some heat banks don't even have overflows.
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I determine You need:
1. a new boiler. 2. a rad in the attic above the cold water tanks 3. better hot water pressure for high pressure showers.
Solution:
1. a new "system boiler" that has the pump and expansion vessel incorporated. 2. a DHW only heat bank that has a boiler coil incorporated, to give: a) high pressure hot and cold supplies, b) a pressurised CH system that will allow your radiator in the loft
The above will allow rads and hot water anywhere in the house. You can start by installing a heat bank only, and then the boiler and rad when money is available.
http://www.heatweb.com http://www.albion-online.co.uk http://www.range-cylinders.co.uk (look at thermal storage [heat banks]) http://www.discountedheating.co.uk
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lots of useful stuff, so I just want to say thanks for your help
cheers
ian
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